I did both NCT and NHS antenatal classes when pg with DS. From my experience, the NCT classes are worth every single penny. Lots of information, lots of time for questions, and what I really liked, lots of effort to show the dads how to get involved. My class was nice and relaxing, and the teacher made an effort to tailor it as much as possible. However, the breastfeeding part of it wasn't too great, but that was really because of the bf councillor that taught it, she was rubbish.
But most importantly, the network of friends that you build in the NCT classes is invaluable - we are still friends 2 years on, meeting regularly, going on holidays together and helping each other out. For that alone I would say, do NCT, knowing people in the same situation as yourself is so immensely useful.
The NHS classes, on the other hand - cramped, too short (2x 2hrs where I live), hardly time for questions/discussions and no real opportunity to get to know the other people.
But that's just my experience!